The present invention relates generally to power supplies, and more particularly to power supplies comprising a plurality of power supply modules.
Large amounts of electric energy can be stored in very dense packages because of advancements in battery technology. Innovation is continually increasing the density of energy storage devices. While necessary to achieve the volume and mass density of high capacity battery packs, this high energy density combined with battery construction materials presents considerable safety issues. Avoiding spectacular catastrophic events requires a battery management system to monitor the operating parameters of the battery cells and take actions to prevent these parameters from exceeding safety specifications.
Known battery management systems include some form of monitoring circuit to determine the status of the battery cells being managed. The actions taken by these systems in response to the status of the monitored parameters can be classified into several groups.
The simplest action is to disconnect the battery system whenever any cell in the system begins operating outside of its safety specifications. A major drawback to this approach occurs when all cells in the system are not perfectly matched and one cell approaches failure before the others. The failure of this cell will cause the device being powered by the battery pack to fail even if the combined pack is still capable of driving the load. The failure of this cell during charge will halt the charging of other cells and the capacity of the entire pack will be significantly depleted.
Another action is used to avoid premature discontinuation of the charging process in the event that one or more cells begin exceeding their recommended operating parameters before all the cells are charged. In this method, a shunt regulator is used to shunt charging current around the suspect cells while allowing the other cells to fully charge. On discharge, this method still requires the pack to be disconnected in the event of the failure of one cell despite the state of the other cells.
A more sophisticated approach attempts to keep all the cells in a battery pack in balance. This balancing method uses one or more isolated DC-DC convertors. The input terminals to each DC-DC converter are connected to the battery pack terminals while the output is connected through an electronic switching network to a cell that is lagging behind the others. In this way, the DC-DC converter assists the lagging cell allowing it to keep up with the others. A variation of this method employs a charge shuttling scheme that allows charge from other batteries in the pack to be shuttled from strong battery cells to weak battery cells. Since this approach requires the use of multiple DC-DC convertors, the cost of implementing this approach is significantly higher than other simpler approaches.
What is needed is a low-cost solution for preventing the operation of individual power sources outside of their safety specifications while maintaining a desired output.